This invention relates generally to producing optical waveguides by ion exchange; and, in one preferred embodiment relates to producing an optical waveguide having a grating impressed therein.
Interest in the use of ion-exchanged glass waveguides for integrated optics has increased considerably recently, since the operation of optical glass waveguides is passive and does not allow adjustment after production. To produce optical fiber compatible waveguide devices by an ion exchange technique, two-step processes are generally used. In these processes, waveguides are formed by the exchange of the original ions in the glass (typically sodium ions Na+) to ions increasing the refractive index (such as K+, Ag+, Cs+, Rb+, Li+ or TI+ ions) through a narrow opening in the ion exchange mask, and by using salt melts or a silver film as an ion source. In the second step, thermal treatment or ion exchange in an NaNO3 melt modifies the refractive index profile of the waveguide to obtain better coupling to an optical fibre. A description of the basic principles of ion exchange are found in an article entitled Ion-Exchanged Glass Waveguides: A Review, R. V. Ramaswamy, Journal of Lightwave Technology, Vol. 6, No. 6, June 1988, P. 984.
An early teaching of making waveguides in a substrate is found in U.S. Pat. No. 4,793,675 in the name of Handa, assigned to Canon Kabushiki Kaisha of Japan. Handa discloses a method of making a element having a light waveguide in which the input-output area through which light is input or output is made into a light waveguide of a high threshold value of optical damage formed by outside diffusion of lithium oxide or by ion exchange.
Further work in the field of producing optical waveguides by ion exchange on a glass substrate can be found in U.S. Pat. No. 5,160,523 in the name of Honkanen et al. assigned to Oy Nokia AB, of Helsinki Finland, issued Nov. 3, 1992. In this disclosure, in order to alleviate the tolerances allowed for the ion exchange technique, the waveguides are formed in the invention by diffusing ions which increase the refractive index away from a waveguide formed previously on the glass substrate and being wider than the optical waveguides to be produced by using the ion exchange technique and a positive type ion exchange mask.
As with optical fibres, there is increasing interest in fabricating devices within a monolithic block of glass, comprising optical waveguides; however this has continued to be difficult. A process has recently been disclosed by Nippon Sheet Glass Co., Ltd. of Japan that relates to fabricating a grating on glass using laser machining. A diffusion process is first performed wherein molten salt comprising AgNO3+NaNO3 is diffused at a temperature of 300 degrees Celsius for a duration ranging from 1 minute to 80 hours in air. Laser machining is later performed using a phase mask to etch a grating within the material. Laser ablation results in regions wherein the waveguide material is etched away creating index differences between ablated regions consisting of air, and the adjacent unablated material interfaces.
In accordance with one aspect of this invention, a method is described of providing a grating within a monolithic waveguide, by first performing an ion exchange process, wherein, for example Na+ ions are exchanged with Ag+ ions to provide a waveguide within glass or another substrate material. According to known techniques of writing or impressing optical gratings in germanium doped optical fibre, as are described in U.S. Pat. No. 5,327,515, 5,104,209, 5,216,739, 4,725,110, and, 4,800,950 we have discovered that a grating can be impressed by, for example interfering two beams within the Ag+ ion exchanged waveguide.
In a preferred embodiment, prior to the ion exchange process, the material to be used is comprised as follows: